I don't really get from warning message which object is causing it. Xcode simply doesn't say in which line that object is. Any advices how to find out where this warning is coming from?
– olyvJan 1 '18 at 15:40

10 Answers
10

Before Swift 4, the compiler made some Swift declarations automatically available to Objective-C. For example, if one subclassed from NSObject, the compiler created Objective-C entry points for all methods in such classes. The mechanism is called @objc inference.

In Swift 4, such automatic @objc inference is deprecated because it is costly to generate all those Objective-C entry points. When "Swift 3 @objc Inference" setting is set to "On", it allows the old code to work. However, it will show deprecation warnings that need to be addressed. It is recommended to "fix" these warnings and switch the setting to "Default", which is the default for new Swift projects.

Thank you Evgenii. Is this a long term Solution?
– DaleKJun 6 '17 at 9:41

5

@DaleK yes, I believe so. According to the Swift proposal I mentioned in my answer, the objc inference is deprecated. The setting "Swift 3 objc Inference" is only present in the projects that were migrated from older versions of Swift. If one creates a new project, the setting is no longer present, which means that objc inference is off. It is recommended to address any objc inference warnings and set it to "Off".
– EvgeniiJun 6 '17 at 11:11

@courteouselk, according to the Swift proposal, one can set SWIFT_DEBUG_IMPLICIT_OBJC_ENTRYPOINT environmental variable to values from 1 to 3 to see the usages of the Objective-C entry points in the logs.
– EvgeniiJun 7 '17 at 5:12

14

Just to add - You need to do this for all the build tagets, not just the project.
– KrivvenzAug 4 '17 at 14:35

- What is @objc inference? What is going on?

In Swift 3, the compiler infers @objc in a number of places so you wouldn't have to. In other words, it makes sure to add @objc for you!

In Swift 4, the compiler no longer does this (as much). You now must add @objc explicitly.

By default, if you have a pre-Swift 4 project, you will get warnings about this. In a Swift 4 project, you will get build errors. This is controlled via the SWIFT_SWIFT3_OBJC_INFERENCE build setting. In a pre-Swift 4 project this is set to On. I would recommend to set this to Default (or Off), which is now the default option on a new project.

It will take some time to convert everything, but since it's the default for Swift 4, it's worth doing it.

- How do I stop the compiler warnings/errors?

There are two ways to go about converting your code so the compiler doesn't complain.

One is to use @objc on each function or variable that needs to be exposed to the Objective-C runtime:

@objc func foo() {
}

The other is to use @objcMembers by a Class declaration. This makes sure to automatically add @objc to ALL the functions and variables in the class. This is the easy way, but it has a cost, for example, it can increase the size of your application by exposing functions that did not need to be exposed.

@objcMembers class Test {
}

- What is @objc and why is it necessary?

If you introduce new methods or variables to a Swift class, marking them as @objc exposes them to the Objective-C runtime. This is necessary when you have Objective-C code that uses your Swift class, or, if you are using Objective-C-type features like Selectors. For example, the target-action pattern:
button.addTarget(self, action:#selector(didPressButton), for:.touchUpInside)

- Why would I not mark everything @objc?

There are negatives that come with marking something as @objc:

Increased application binary size

No function overloading

Please keep in mind that this is a very high-level summary and that it is more complicated than I wrote. I would recommend reading the actual proposal for more information.

Are there other ways to add action to button? If @objc depricated what we must use?
– AznixSep 21 '17 at 6:35

5

@Stefan yes, there may be quite a bit to convert. Split it up into stages. Leave SWIFT_SWIFT3_OBJC_INFERENCE at On. Convert to Swift 4. Then tackle the @objc stuff. To make it simple, follow basic rules: IF the Swift class is used in Objc-C code (via bridging header), use @objcMembers, otherwise, one-by-one add @objc. Simply use Xcode search to find out whether the Swift class is called from any .m file. This should make the conversion relatively painless.
– kgaidisSep 26 '17 at 13:01

@DaleK this should be accepted answer. Suppressing warnings and making things work as they were in Swift 3 is an option, but IMHO not the best one. It's important to understand why did @objc change in Swift 4 and then make decision to fix project and keep it the same.
– derpoliukNov 17 '17 at 11:07

what step 3 is all about? You could add some descriptions :)
– ShialSep 20 '17 at 4:43

And in my case, I get this warning, but no code is pointed to. There are two methods marked @objc and they seem like the only ones that need it. I changed it to Default and still get the warnings during compile.
– Maury MarkowitzMar 30 '18 at 15:38

I had this warning with "Swift 3 @objc Inference" = "Default" setting. Then I realized that is was set for the Project - not for the target. So, make sure that you have "Default" setting in your target to get rid of the warning.

After migrating to Swift 4, and setting "Swift 3 @objc Inference" to default, certain features of my project stopped working.
It took me some debugging and research to find a solution for this.
According to my best knowledge, here are the options:

Enable "Swift 3 @objc Inference" (only works if you migrated an existing project from Swift 3)

Mark the affected methods and properties as @objc

Re-enable ObjC inference for the entire class using @objcMembers

Re-enabling @objc inference leaves you with the warnings, but it's the quickest solution. Note that it's only available for projects migrated from an earlier Swift version.
The other two options are more tedious and require some code-digging and extensive testing.

I'm an occasional iOS dev (soon to be more) but I still couldn't find the setting as guided by the other answer (since I did not have that Keychain item the answer shows), so now that I found it I thought I might just add this snapshot with the highlighted locations that you will need to click and find.

Start in the upper left

Choose the project folder icon

Choose your main project name below the project folder icon.

Choose Build Settings on the right side.

Choose your project under TARGETS.

Scroll very far down (or search for the word inference in search
text box)

All you need just run a test wait till finish, after that go to Build Setting, Search in to Build Setting Inference,
change swift 3 @objc Inference to (Default). that's all what i did and worked perfect.